There has been increasing application of luminescence based sensors in areas such as environmental monitoring, biochips, DNA chips, bioluminescence, chemiluminescence and many others. The sensors typically comprise a substrate having a luminescence emitter in optical contact therewith. The emitter may of itself transmit luminescence into the substrate or may require to be activated in some way, either by incident light or by some chemical reaction. There are various forms of luminescence: phosphorescence which is long lived light transmission, fluorescence which is short lived, chemiluminescence where two chemicals react and finally bioluminescence. Within the present specification the term “luminescence” is intended to encompass all such forms of luminescence. Many chemical sensors and biosensors are based on the luminescence emitted from thin layers or patterned arrays of fluorophores deposited on a dielectric interface such as a waveguide surface or a transparent substrate. One of the key issues in these sensors is the efficiency of the luminescence collection.
A typical configuration in many luminescence-based sensor applications involves a thin luminescent film or spot deposited onto a planar substrate. An example of such a configuration is FIG. 1, which shows a substrate 100 having a luminescent spot 110 deposited thereon. A detector 120 is provided under the substrate 100 and is adapted to detect light 130 that is transmitted by the spot into the substrate and which passes out of the substrate. It will be noted that the detector is positioned directly under the spot 110 so as to detect light that passes normally (i.e. undeviated by refraction at the interfaces) through the substrate. Within the present specification the element containing the substrate and the luminescent layer or spot will be referred to as the sensing element or sensor chip. The sensor chip is considered to be designed independently of the sensor system in which it is to be incorporated.
A majority of luminescence-based sensor systems employ rather inefficient techniques for the collection of luminescence emitted by a thin sensing film or molecules attached to a surface. A number of authors have developed new ways of dealing with the issue of low luminescence intensity emitted by systems under study. Liebermann et al. [T. Liebermann and W. Knoll. Surface-plasmon field-enhanced fluorescence spectroscopy. Colloids and Surfaces, A: Physicochemical and Engineering Aspects 171:115-130, 2000.] exploited the enhancement of the amplitude of the excitation light in the close vicinity of a metal surface provided by the efficient excitation of the surface plasmon wave. Blair & Chen [S. Blair and Y Chen. Resonant-enhanced evanescent-wave fluorescence biosensing with cylindrical optical cavities. Applied Optics, 40(4):570-581, 2001] showed that luminescence of molecules can be enhanced by the use of planar cylindrical resonant optical cavities.
It is also known to incorporate metal coatings or metal nanoparticles into a sensor and the incorporation of these materials can have a very positive influence on the intensity of luminescence emitted by molecules located in their close vicinity. Enhancement of the quantum yield in the order of 100-1000 has been reported. Although these developments are certainly valuable for the improvement of the performance of the luminescence-based chemical sensors and bio-sensors, they do not address the issue of efficiency of the luminescence collection.
Polerecky L et al (Applied Optics 39 (22): 3968-3977 Aug. 1, 2000) have described a theory of radiation from dipoles embedded inside an arbitrary multilayer system . They derive explicit expressions for the angular distribution of the electromagnetic field and intensity radiated by the dipole into the surrounding media. Using this theoretical analysis they conclude consequences for optimisation of optical chemical sensors and biosensors based on luminescence emission, specifically that as a large proportion of the luminescence is radiated into the higher refractive index substrate and due to total internal reflection at the glass/air interface is guided along the glass-slide, better results should be provided by detecting the luminescence at the edge of the glass-slide. Although this technique facilitates the detection of the modes that normally propagate along the glass slide towards the edge, the detection is not optimised, as only those modes propagating within a narrow angular range Δφ, as shown in FIG. 2, are detected. In order to maximise this fraction, detectors would have to be placed all around the substrate which is not feasible in most practical applications.
There therefore exists a need for a method and sensor for detecting a luminescence signal that is based on the understanding that a large proportion of luminescent light that is radiated into a substrate to which the material is attached is not detected.